xv68k:  Don't clobber --pid parameter for later -A

Let `xv68k --pid=123 -A` use a pid of 123, not the real pid.
xv68k:  Const-qualify load_argv()'s *argv
xv68k:  Const-qualify execute_68k()'s argv
xv68k:  Check for argc == 0 early
xv68k:  Don't return from BAD_USAGE()
xv68k:  Call exit(), not return, in option parsing
xv68k:  Factor out get_options()
xv68k:  Rewrite option processing via get_option()
Merge branch 'xv68k-options'